Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dpm309 on March 14, 2018, 12:53:33 pm

Title: Univalve
Post by: dpm309 on March 14, 2018, 12:53:33 pm

Working on a dead THD Univalve that was playing fine and then a static/sweep sound came out of the speakers and the amp went dead. The noise was like when I blew the PT on my Tweed Deluxe clone, twice. Tubes light up but I am getting no power at all from the HV secondaries and no B+ voltage on the output of the bridge rectifier (BR1). Fuses are all good. Tried checking the resistance on the primary and secondaries as per the amp debugging page and showed to be within the recommendations. Before I order a new one, is there anything else I should be looking at? What could of caused the sudden failure of the PT? The schematics can be found on Doug's schematic page.
Thanks

Title: Re: Univalve
Post by: sluckey on March 14, 2018, 01:34:51 pm

Do you measure 120VAC ***ACROSS*** the actual primary leads? IOW, is 120VAC actually getting to the PT primary?

Title: Re: Univalve
Post by: dpm309 on March 14, 2018, 02:02:33 pm

Yes, I am getting 120VAC to the PT primary. Rechecked the HV fuse and it is blown. Could not tell by a visual inspection. Put in another .25amp SB fuse and seems to be working. Checked the amp out with the light limiter and there does not seem to be any shorts. Will let it run awhile to see if it did the trick.

I can't believe I didn't double check before posting. Will always check for continuity on the fuses in the future!
Thanks

Title: Re: Univalve
Post by: pompeiisneaks on March 16, 2018, 05:17:40 pm

Quote from: dpm309 on March 14, 2018, 02:02:33 pm

Yes, I am getting 120VAC to the PT primary. Rechecked the HV fuse and it is blown. Could not tell by a visual inspection. Put in another .25amp SB fuse and seems to be working. Checked the amp out with the light limiter and there does not seem to be any shorts. Will let it run awhile to see if it did the trick.

I can't believe I didn't double check before posting. Will always check for continuity on the fuses in the future!
Thanks

Just be aware if it blows again, it's a hint something else is wrong, but sometimes after years of use and occasional wiring spikes at the wall, they can just 'go' sometimes

~Phil

Title: Re: Univalve
Post by: dpm309 on March 17, 2018, 11:02:20 am

Have been running the amp for awhile with no problems. Turns out that the amp was switched to high power with a 6V6 (THD says to use Lo power only with a 6V6) and blew shortly after that. I am guessing a power surge caused by switching from low to high power without switching off the standby caused this. Better than blowing the transformers.

Title: Re: Univalve
Post by: PRR on March 17, 2018, 07:23:28 pm

> Lo power only with a 6V6

Put in a switch to sense the height of the bottle. If under 2.8", switch to lo-power.

Or just accept the easy(?) work.